Conventionally, each compressor rotor stage of a gas turbine engine comprises a plurality of radially extending blades mounted on a rotor disc. The blades are mounted on the disc by inserting a root portion of the blade in a complementary retention groove in the outer face of the disc periphery. To ensure a smooth radially inner surface for air to flow over as it passes through the stage, annulus fillers are used to bridge the spaces between adjacent blades. Typically, seals between the annulus fillers and the adjacent fan blades are also provided by resilient strips bonded to the annulus fillers adjacent the fan blades.
Annulus fillers of this type are commonly used in the fan stage of gas turbine engines. The fillers may be manufactured from relatively lightweight materials and, in the event of damage, may be replaced independently of the blades. The fillers are spaced from the rotor disc and define a hollow cavity that is separated from the air flow passage by the smooth inner surface defined by the annulus filler.
It is known to provide annulus fillers with features for removably attaching them to the rotor disc. For example, it has been proposed to provide annulus fillers with axially spaced hook members, the hook members sliding into engagement with respective parts of the rotor disc. FIG. 1 shows an example of such an annulus filler viewed from the side, and FIG. 2 shows the annulus filler fitted to the rotor disc as viewed in transverse cross-section.
In use, the upper surface or lid 2 of the annulus filler 1 bridges the gap between two adjacent fan blades 3 (one of which is shown in outline if FIG. 2) and defines the inner wall of the flow annulus of a fan stage. The annulus filler 1 is mounted on a fan disc 4 by two hook members 5, 6 respectively towards the forward and rearward ends of the annulus filler 1. The hook members are configured to engage with outwardly directed hooks provided on the fan disc 4. The annulus filler is also attached to a support ring 7 by a retention flange 8 provided at the forward end of the annulus filler. Along its rear edge, the annulus filler is provided with a rear lip 9 which is configured to fit under a rear fan seal 10 located axially behind the rotor disc 4 to limit deflection under running conditions. Similarly, the front edge of the annulus filler defines a front lip 11 which is configured to fit under a spinner fairing 12 located axially ahead of the annulus filler. The two opposed side faces 13, 14 of the annulus filler are provided with respective seal strips (not shown) and confront the aerofoil surfaces of the adjacent fan blades 3 in a sealing manner.
As illustrated in more detail in FIG. 3, the retention flange 8 carries a forwardly extending spigot or pin 15. The spigot or pin 15 is arranged for engagement within a corresponding aperture or recess provided in the support ring 7. At a position circumferentially adjacent the spigot or pin 15, the retention flange is also provided with a mounting aperture 16 which is arranged for co-alignment with a corresponding mounting aperture (not shown) provided through the support ring 7. The co-aligned mounting apertures are sized to receive a mounting bolt. Thus, it will be appreciated that the retention flange 8 is pinned and bolted to the front support ring 7.
FIG. 4 illustrates the typical form of the rear hook member 6, as viewed from behind. As can be seen, the hook member defines an arcuate channel 17. The channel 17 is curved in such a manner as to be centred on the rotational axis of the engine (not shown), and cooperates with a correspondingly arcuate hook on the rotor disc 4. The front hook member 5 has a similar arcuate configuration.
A problem which has been experienced with prior art annulus fillers of the general type described above is that of reliable installation during engine assembly. As will be appreciated by those of skill in the art, the annulus filler must be fitted after the radially extending fan blades have been attached to the rotor disc. This means when the fitter then installs the annulus fillers between adjacent blades, his or her line of sight is obstructed by the presence of the fan blades. Also, the unitary construction of the annulus filler exacerbates this problem, because the filler lid 2 also obstructs the fitter's view when attempting to engage the hook members 5, 6 with the rotor disc 4. Misassembly of the rear hook member 6 has been found to be a particular problem in this regard and has been attributed to the release of annulus fillers in operation.
Annulus fillers of the prior-art type described above are self-loading in the sense that, as a rotating component, the majority of forces on the filler are generated by its own mass. This can be modelled as a near to radial force acting through the centre of gravity of the annulus filler. However, in the event of a bird-strike, or a fan blade otherwise becoming detached from the rotor (i.e. a so-called “fan-blade-off” event), the blades can apply tangential pushing forces to the adjacent annulus fillers thereby tending to pinch the annulus fillers between the blades as the blades pivot tangentially in their retention grooves. This can cause the annulus fillers to become detached from the rotor. In this regard, it is to be noted that a bird-strike or fan-blade-off event creates substantial imbalance in the rotor, and so even the remaining fan blades can deflect considerably due to their tips impinging on the outer casing surrounding the rotor. Thus it is not unknown to lose annulus fillers from circumferential positions well away from the primary release blade.
It has been found that the above-described configuration of annulus filler can increase the likelihood of the filler failing under the action of the tangential forces applied to it by the adjacent fan blades. Due to the curved nature of the interface between the hook members 5, 6 on the annulus filler and the cooperating hooks formed on the rotor disc 4, the natural tendency of an annulus filler pushed from the side by an adjacent fan blade is to move rotationally relative to the disc, about the engine axis. However, because the front end of the filler is securely fixed by being pinned and bolted to the support ring, the front region of the filler is not permitted to deflect in this manner. The result is that the annulus filler becomes twisted along its length, which can lead to the filler fracturing between the retention flange 8 and the front hook member 5. As will be appreciated, failure of annulus fillers in this manner is problematic as it increases the amount of shrapnel moving around inside engine during a bird-strike or fan-blade-off event, which can have serious consequences.